In the field of mobile communication systems, the need for higher data transmission speeds is increasing. To address this need, work on communication standards achieving higher communication speeds than High Speed Downlink Packet Access (HSDPA) has been proceeding. One example of a communication standard capable of achieving such high-speed data communications is Long Term Evolution (LTE) on which the Third Generation Partnership Project (3GPP) is working to standardize.
In LTE, some of the functions conventionally incorporated into a radio network controller (RNC) are incorporated into a base station (Evolved Universal Terrestrial Radio Access Network (E-UTRAN) NodeB (eNB)) thereby greatly enhancing the functionality of the base station. In executing a handover, for example, there are instances in which a downlink signal buffered at the source base station (hereinafter referred to as eNB1) with which the mobile station as a user terminal has been communicating via radio until the execution of the handover has not yet been transmitted to the mobile station. If there is any such downlink signal, the eNB1 transfers the downlink signal to another base station (hereinafter referred to as eNB2) to which the mobile station is connected after the execution of the handover. In this case, the eNB2 receives downlink signals not only from the eNB1 but also from an upper node. Of the downlink signals received over the two different paths, the eNB2 first transmits any downlink signal received from the eNB1 to the mobile station in order to preserve the sequence integrity of the data to be delivered to the mobile station. After that, the eNB2 transmits the downlink signal directly received from the upper node to the mobile station (refer, for example, to 3GPP TS 36.300, v8.7.0, Sections 10.1.2.1 and 10.1.2.1.2).
Another technique has been proposed for transmitting such downlink signals to the mobile station when a handover occurs while any such downlink signal is buffered at the base station (refer, for example, to Japanese Laid-open Patent Publication No. 2007-266790). According to this technique, the base station that holds the packets yet to be transmitted to the mobile station transmits some of the packets directly to the mobile station while transferring the other packets to the handover destination base station. Then, these other packets received at the handover destination base station are transmitted to the mobile station.
While the yet-to-be-transmitted downlink signal is being transferred from the eNB1 to the eNB2 due to the execution of the handover, if a new handover occurs, the above process is repeated. As a result, after the data transfer from the eNB1 to the eNB2 is completed, the yet-to-be-transmitted downlink signal is further transferred to still another base station (hereinafter called the eNB3) to which the mobile station is connected after the execution of the new handover. Further, the upper node that first switched the destination of the downlink signal from the eNB1 to the eNB2 next switches the destination from the eNB2 to the eNB3.